docs: def get_centroids(self, ridx):
          """
          :returns: array of centroids for the given rupture index
          """
          centroids = []
          with h5py.File(self.source_file, "r") as hdf5:
              for idx in ridx:
                  trace = "{:s}/{:s}".format(self.idx_set["sec"], str(idx))
                  centroids.append(hdf5[trace + "/Centroids"].value)
          return numpy.concatenate(centroids)